1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to an improved method for making a CPP sensor that contains a ferromagnetic alloy, like a Heusler alloy, that requires high-temperature post-deposition annealing.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically formed of Cu or Ag. One ferromagnetic layer, typically called the “reference” layer, has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic “pinning” layer, and the other ferromagnetic layer, typically called the “free” layer, has its magnetization direction free to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In a magnetic recording disk drive CPP-GMR read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
The fixed or pinned ferromagnetic layer in a CPP-GMR sensor used in read heads may be a single pinned layer (sometimes called a “simple” pinned layer) or part of an antiparallel (AP) pinned structure. In a simple pinned structure the pinned layer has its magnetization pinned by being exchange-coupled to an antiferromagnetic pinning layer, which is typically a Mn alloy like IrMn. In an AP-pinned structure first (AP1) and second (AP2) ferromagnetic layers are separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP1 layer, which is in contact with the antiferromagnetic layer on one side and the nonmagnetic APC layer on the other side, is the pinned layer. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's electrically conductive spacer layer on the other side, is the reference layer. The AP-pinned structure minimizes magnetostatic coupling between the reference layer and the CPP-SV free ferromagnetic layer.
The materials making up the ferromagnetic layers are typically crystalline alloys of CoFe or NiFe. However, Heusler alloys, which are chemically ordered alloys like Co2MnX (where X is one or more of Ge, Si, Sn, Ga or Al) and Co2FeZ (where Z is one or more of Ge, Si, Al, Sn or Ga), are known to have high spin-polarization and result in an enhanced magnetoresistance and are thus desirable ferromagnetic materials to use in one or both of the reference layer and free layer. Heusler alloys are usually deposited directly on a layer of a crystalline ferromagnetic material such as Co or CoFe. Other materials with high spin-polarization are non-Heusler alloys of the form CoFeX (where Xis one or more of Ge, Al, Si or Ga).
Heusler alloys and non-Heusler alloys of the form CoFeX (where X is one or more of Ge, Al, Si or Ga) require significant high-temperature post-deposition annealing to achieve chemical ordering and high spin-polarization. However, at high annealing temperatures the Mn in the antiferromagnetic pinning layer diffuses into other layers in the sensor stack, which reduces the pinning strength and alters the desired composition of the Heusler alloy. This results in degraded sensor performance. The insertion of an amorphous CoFeB ferromagnetic layer at the interface between the antiferromagnetic pinning layer and the ferromagnetic pinned layer has been suggested to block the diffusion of Mn. However, after post-deposition annealing the sensor with the CoFeB interface layer exhibits only marginal improvement in magnetoresistance.
What is needed is an improved method for making a CPP-GMR sensor that has one or more ferromagnetic layers formed of a material, like a Heusler alloy, that requires high-temperature post-deposition annealing.